2Chapter Objectives Understand concepts of normalization Learn how to normalize tablesUnderstand normalization and database design issues2
3Database Tables and Normalization Normalization is a process for assigning attributes to entities.It reduces data redundancies.An un-normalized relation (table) stores redundant data, which can cause insertion, deletion, and modification anomalies.In simple words: Normalization means keeping a single copy of data in your database.Normalization theory provides a step by step method to remove redundant data and undesirable table structures.4
4Normal FormsTables are normalized by applying rules to create a series of normal forms:First normal form (1NF)Second normal form (2NF)Third normal form (3NF)Boyce/Codd normal form (BCNF)Fourth normal form (4NF)Projection Join normal form (PJNF, aka 5NF)A table or relation in a higher level normal form always confirms to lower level normal forms.
5Normal FormsPJ/NF (5NF) Relations4NF RelationsBCNF Relations3NF Relations2NF Relations1NF RelationsWhile higher level normal forms are available, normalization up to BCNF is often found to be adequate for business data.
6First Normal FormA relation is in 1NF if all underlying domains contain atomic values only, i.e., the intersection of each row and column contains one and only one value.The relation must not contain repeating groups.PNo PName ENo EName Jcode ChgHr Hrs1 Alpha 101 John Doe NE $65 20105 Jane Vo SA $80 15110 Bob Lund CP $60 402 Beta 101 John Doe NE $65 20108 Jeb Lee NE $65 15106 Sara Lee SA $80 203 Omega 102 Beth Reed PM $125 20105 Jane Vo SA $80 10Is the above relation in 1NF?9
7First Normal FormThe previous relation can be converted into first normal form by adding Pno and Pname to each row.PNo PName ENo EName Jcode ChgHr Hrs1 Alpha 101 John Doe NE $65 201 Alpha 105 Jane Vo SA $80 151 Alpha 110 Bob Lund CP $60 402 Beta 101 John Doe NE $65 202 Beta 108 Jeb Lee NE $65 152 Beta 106 Sara Lee SA $80 203 Omega 102 Beth Reed PM $125 203 Omega 105 Jane Vo SA $80 10What is the primary key in this relation?Do you see redundant data in this table?What anomalies could be caused?
8Functional Dependency Revisited If A and B are attributes (or group of attributes) of a relation R, B is functionally dependent on A (denoted A B), if each value of A in R is associated with exactly one value of B in R.A is called a determinant.Consider the relationStudent (ID, Name, Soc Sec Nbr, Major, Deptmt)Assume a department offers several majors, e.g. INSY department offers, INSY, MASI, and POMA majors.How many determinants can you identify in Student?
9Functional Dependency Revisited A Dependency diagramIDNameSoc_Sec_NbrMajorDept
10Functional Dependency Revisited Full functional dependencyAttribute B is fully functionally dependent on attribute A if it is functionally dependent on A and not functionally dependent on any proper subset of A.This becomes an issue only with composite keys.Transitive dependencyA, B and C are attributes of a relation such that A B and B C, then C is transitively dependent on A via B (provided that A is not functionally dependent on B or C)
11Second Normal Form Dependency diagram for Project PNo PName ENo EName JCodeChgHrHrs
12Second Normal Form A relation is in 2NF if: It is in 1NF and every nonkey attribute is fully dependent on the primary key, i.e., no partial dependency.A nonkey attribute is one that is not a primary key or part of a primary key.We create new relations that are in 2NF through projection of the original relation.Project(PNo, PName)Employee(ENo, EName, Jcode, ChgHr)Charge(PNo, ENo, Hrs)14
14Second Normal Form Tables in 2NF Charge PNo ENo Hrs 1 101 20 Project ProjectPNo PName1 Alpha2 Beta3 OmegaEmployeeENo EName JCode ChgHr101 John Doe NE $65102 Beth Reed PM $125105 Jane Vo SA $80106 Sara Lee SA $80108 Jeb Lee NE $65110 Bob Lund CP $60
15Second Normal FormNote that the original relation can be recreated through natural join of the new relation.Thus, no information is lost in the process of creating 2NF relations from a 1NF relation. This is called nonloss decomposition.If a relation that is in 1NF has a non composite primary key (i.e., the primary key consists of a single attribute) what can you say about its status with regard to 2NF?Do you see any redundant data in the tables that are in 2NF?What anomalies could be caused by such redundancy?
16Third Normal Form A relation is in 3NF if: It is in 2NF and every nonkey attribute is nontransitively dependent on the primary key (i.e., no transitive dependency).Relation Employee has a transitive dependency:ENo JCode ChgHrEmployee can be replaced by two relations, that are in 3NF:Employee(ENo, EName, Jcode)Job(JCode, ChgHr)16
18Third Normal Form Tables in 3NF Charge PNo ENo Hrs 1 101 20 Project ProjectPNo PName1 Alpha2 Beta3 OmegaEmployeeENo EName Jcode101 John Doe NE102 Beth Reed PM105 Jane Vo SA106 Sara Lee SA108 Jeb Lee NE110 Bob Lund CPJobJcode ChgHrCP $60NE $65PM $125SA $80
19Boyce-Codd Normal Form A relation is in BCNF ifevery determinant is a candidate key.A determinant is an attribute (combination of attributes) on which some other attribute is fully functionally dependent.BCNF is a special case of 3NF.The potential to violate BCNF may occur in a relation that:contains two (or more) composite candidate keys,these keys overlap and share at least one attribute.Thus, if a table contains only one candidate key or only non-composite keys, then 3NF and BCNF are equivalent.18
21Decomposition of Table Structure to Meet BCNF Figure 4.8
22Boyce-Codd Normal Form Consider the following example:The members of a recruiting team interview candidates on a one-to-one basis. Each member is assigned a particular room on a given date. Each candidate is interviewed only once on a specific date. He/she may return for follow up interviews on later dates.Interview (CID, IDate, ITime, StaffID, RmNo)CID IDate ITime StaffID RmNoC :00 S01 B107C :00 S01 B107C :00 S05 B108C :00 S06 B108
23Boyce-Codd Normal Form This relation has following functional dependencies:CID, IDate ITime, StaffID, RmNoStaffID, IDate, ITime CID, RmNoRmNo, Idate, Itime StaffID, CIDStaffID, IDate RmNoThis relation does not have any partial or transitive dependencies on the primary key (CID, IDate)It is not in BCNF because (StaffID, Idate) is a determinant but not a candidate key.The new relations in BCNF are:Interview (CID, IDate, ITime, StaffID)Room(StaffID, IDate, RmNo)
25Fourth Normal Form A table is in 4NF if it is in 3NF andhas no multiple sets of multivalued dependencies.Consider the following example:Each course is taught by many teachers and requires many texts.CTXU (Unnormalized)Course Teacher TextPhysics Green Basic MechanicsBrown Intro to OpticsMath White Modern AlgebraIntro to CalculusCTXN (Normalized)Course Teacher TextPhysics Green Basic MechanicsPhysics Green Intro to OpticsPhysics Brown Basic MechanicsPhysics Brown Intro to OpticsMath White Modern AlgebraMath White Intro to Calculus30
26Fourth Normal FormCTXN is in BCNF, because it is all key and there are no other functional dependencies.It, however, has redundant data that could cause update anomalies.This table shows two multivalued dependencies:Each course has a defined set of teachers andCourse TeacherEach course has a defined set of textbooks.Course TextMVDs can exist only when the relation has at least three attributes.An FD is a special case of MVD when the set of dependent values has a single value.
27Fourth Normal Form Tables in 4NF CX CT Course Text Course Teacher Physics GreenPhysics BrownMath WhiteCXCourse TextPhysics Basic MechanicsPhysics Intro to OpticsMath Modern AlgebraMath Intro to Calculus
28Conversion to 4NF Figure 4.15 Set of Tables in 4NF Figure 4.14 Multivalued Dependencies
29Normalization and Database Design Normalization should be part of the design processE-R Diagram provides macro viewNormalization provides micro view of entitiesFocuses on characteristics of specific entitiesMay yield additional entitiesDifficult to separate normalization from E-R diagrammingBusiness rules must be determinedNormalization purity is difficult to sustain due to conflict in:Design efficiencyInformation requirementsProcessing
30DenormalizationNormalized (decomposed) tables require additional processing, thus reducing system speed.Sometimes normalization is not done keeping in mind processing speed requirements and practical aspects of the situation.A good example is: storing Zip code and City as attributes in a Customer relation violates 3NF because City is transitively dependent on Cust ID via Zip Code.Why should we not create a separate relation ZIP (ZipCode, City)?32